Cryptography Reference
In-Depth Information
(b) AES is under great scrutiny from the cryptographic research community.
Provide a 'layman's guide' to the latest cryptanalytic attacks that have been
announced against AES and comment on their practical relevance.
(c) Suppose that one day an announcement is made that AES is too insecure
for widespread use. What do you think the likely reaction will be from the
various communities that this will affect?
10
. If a block cipher is used in ECB mode then it is at least theoretically possible to
conduct block frequency analysis.
(a) Suggest some circumstances, based on particular applications of encryp-
tion, which might result in it being possible for an attacker to conduct a
practical block frequency analysis attack on a block cipher used in ECB
mode.
(b) Can block frequency analysis still be conducted if the block cipher used is
not known to an attacker?
11
. At least in theory, the design of a block cipher is generally made more
secure by:
• increasing the number of rounds used in a block cipher;
• using a block cipher with a larger block size.
(a) For each of these techniques, explain why it might be more secure.
(b) For each of these techniques, what are the associated practical
disadvantages?
(c) For each of these techniques, explain which of the three significant
problems with ECB mode that we identified in Section 4.6.1 might be
reduced.
(d) Explain why it is inadvisable to apply either of these techniques to a
published (standardised) block cipher such as AES.
12
. More and more modes of operation of a block cipher are being proposed
over time.
(a) What is the purpose of having different modes of operation of a block
cipher?
(b) Why do you think there has been an increase in the number of modes of
operation of a block cipher being proposed and standardised?
13
. There is nothing, in theory at least, to prevent modes of operation from being
applied to public-key encryption algorithms, which also process plaintexts in
blocks of bits. Which of themodes of operation that we described in Section 4.6
could work if we use a public-key encryption algorithm instead of a symmetric
encryption algorithm?
14
. Many modes of operation of a block cipher essentially convert the block cipher
into a stream cipher by using the encryption process of a block cipher as a
keystream generator. This requires the output of the block cipher (ciphertext)
to appear to be randomly generated. Explain why this is a reasonable (indeed
desirable) property of a block cipher.
